In an industrial material field, an environment field, a pharmaceutical field, and a bio field, light is irradiated to a sample and transmitted light or scattered light thereof or fluorescence generated from the sample by irradiation of light is detected and measured by a light quantity detection device, for analysis of an included component of the sample. In a semiconductor inspection equipment, light is irradiated to a semiconductor wafer and transmitted light or reflected light thereof is measured in order to inspect wirings formed on the semiconductor wafer or inspect foreign materials on the wafer. Further, for measuring aerosol that floats in the air, a laser is irradiated to the air to measure the scattered light. In the light quantity detection device, a detection range (dynamic range) of a light quantity needs to be widened in addition to detection of a weaker light quantity, for more detailed analysis.
The light quantity detecting method in the related art includes an analog measuring method of acquiring the intensity of an output signal of a light detector by light incident on a light detector and a photon count measuring method of counting an output pulse generated whenever photons are incident on the light detector.
First, the light quantity detection device by the analog measuring method will be described with reference to FIG. 2. The light quantity detecting device by the analog measuring method is configured to integrate an output signal 32 of an analog type light detector 31 into a preset time by an integrator 51, convert an output signal 52 of the integrator into a digital signal by an A/D converter 6, and arithmetically process the digital signal in a CPU 90.
FIG. 2 is a diagram illustrating a waveform of a detector output 32 and a waveform of an integrator output 52. When a light quantity incident on the light detector 31 is strong (a. in the case of a strong light quantity), the output signal 32 of the light detector generates a strong detector output signal while light is incident. In the output signal 52 of the integrator of the signal, the integrator output is increased while light is incident and an integration result thereof is acquired by an A/D converter 6.
Meanwhile, Patent Literature 1 (Japanese Patent Application Laid-Open Publication No. Hei5(1993)-264352) disclosed that an integration time is increased in the case of a weak light quantity by a method of changing the integration time according to a light quantity, an output value of the integrator when light is incident can be significantly larger than an output value of the integrator when no light is incident, and measurement precision can be improved.
Further, as an additional method for solving a problem in which measurement precision deteriorates in the case of the weak light quantity, Patent Literature 2 (Japanese Patent Application Laid-Open Publication No. 2008-249694) discloses a method for preventing accumulation of noise signals less than a threshold value by substituting signals less than the threshold value into a reference value and decreasing a measurable light quantity lower limit value, in a time-of-flight mass spectrometer.
Next, the light quantity detection device by the photon count measuring method will be described with reference to FIG. 3. The light quantity detection device by the photon count measuring method is configured to compare the output signal 32 of the photon count type light detector 3 with the threshold value by using a comparator 61 to output 1 in the case of the threshold value or more and 0 in the case of the threshold value or less, measure the number of pulses of an output signal 62 of the comparator which an output is 1 by using a counter 63, and arithmetically process the measured number of pulses in the CPU 90.
FIG. 3 illustrates a diagram illustrating the waveform of the detector output 32 and a waveform of the comparator output 62. When light incident on the light detector 3 is weak (a. in the case of the weak light quantity), the output signal pulse generated whenever photons are incident on the light detector can be detected with high precision.
Further, Patent Literature 3 (Japanese Patent Application Laid-Open Publication No. Hei9(1997)-178852) discloses a method of detecting the light quantity by using the photon count measuring method in the case of the weak light quantity and detecting the light quantity by the analog measurement method in the case of the strong light quantity by in parallel using the analog measurement method and the photon count measuring method. As such, it is disclosed that the light quantity can be detected in a wide dynamic range by switching the measurement method according to the light quantity.